The present invention relates to a plasma processing apparatus and a plasma processing method, and more particularly relates to a plasma processing apparatus and a plasma processing method suitable for forming a fine pattern in a semiconductor device manufacturing process.
The need for improving the fine pattern manufacturing capability and the processing speed in plasma processing is growing further as integration of semiconductor devices become higher. In order to respond to this need, it is required to decrease the pressure of the processing gas and to increase the plasma density.
In regard to plasma processing apparatuses aiming to decrease the pressure of the processing gas and to increase the plasma density, there presently are: (1) a method which utilizes the electron cyclotron resonance phenomena (hereinafter referred to as ECR) of a microwave (e.g., 2.45 GHz electromagnetic field with a static magnetic field (e.g., 875 G); and (2) a method which utilizes induction coupling processing (hereinafter referred to as ICP) in which a plasma is generated by generating an induced electromagnetic field by exciting a coil using an RF frequency power source.
In a case where a film of the oxide film group is etched using a gas of fluorocarbons, when either of the methods of the ECR described in the above item (1) or the ICP described in the item (2) is employed, it is difficult to increase selectivity of an oxide-film to a base material, for example, Si or SiN since dissociation of the gas progresses excessively.
On the other hand, in a conventional method of generating a plasma by applying an RF frequency voltage between a pair of parallel flat plates, it is difficult to stably discharge under a pressure condition below 10 Pa.
As a countermeasure, there are: (3) a two-frequency exciting method in which a plasma is generated using a high frequency voltage above several tens MHz and bias control of a sample is performed using a low frequency voltage below several MHz, which is disclosed in Japanese Patent Application Laid-Open No. 7-297175 or Japanese Patent Application Laid-Open No. 3-204925; and (4) a magnetron RIE (hereinafter referred to as M-RIE) method which utilizes an action of confining electrons by Lorentz force of electrons by applying a magnetic field B in a direction intersection with a self-bias electron field (E) induced on the surface of the sample, which is disclosed in Japanese Patent Application Laid-Open No. 2-312231.
Further, a method of increasing plasma density under a low pressure condition is described in Japanese Patent Application Laid-Open No. 56-13480. This method obtains a high plasma density under a low pressure condition of 0.1 Pa to 1 Pa by utilizing an electron cyclotron resonance (ECR) effect induced by a microwave of electromagnetic waves (e.g., 2.45 GHz) and a static magnetic field (e.g., 875 gauss).
On the other hand, in the technical field of performing etching processing or film forming processing of a semiconductor material using a plasma, an apparatus is employed having a high frequency power source for accelerating ions in a plasma to a sample table for mounting an object to be processed (for example, a semiconductor wafer substrate, hereinafter referred to as the sample) and an electrostatic attracting film for holding the sample on the sample table by an electrostatic attracting force.
For example, in an apparatus disclosed in the specification of U.S. Pat. No. 5,320,982, a plasma is generated by microwaves and a sample is held on a sample table by an electrostatic force, and using a high frequency power source output having a sinusoidal waveform as a bias electric source, the ion energy incident on the sample is controlled by connecting the power source to the sample table while the temperature of the sample is being controlled by introducing a heat transfer gas between the sample and the sample table.
Further, Japanese Patent Application Laid-Open No. 62 280378 discloses that a distribution of the ion energy incident to the sample can be narrowed by applying a pulse-shaped ion control bias voltage to a sample table for maintaining the electric field intensity between a plasma and an electrode at a constant value. Thereby, it is possible to improve the dimensional accuracy of plasma etching processing and the etching rate ratio of a processed film to a base material by several times.
Furthermore, Japanese Patent Application Laid-Open No. 6-61182 discloses that it is possible to prevent the occurrence of notches by generating a plasma utilizing electron cyclotron resonance and applying a pulse bias having a width of pulse duty of 0.1% or more to a sample.
An example of increasing a plasma density by generating cyclotron resonance using an electromagnetic wave of VHF band and a static magnetic field is described in the Journal of Applied Physics, Japan, Vol. 28, No. 10. However, in this example, by applying a high frequency voltage of 144 MHz to a coaxial central conductor and adding a magnetic field of 51 gauss in parallel to the central conductor, cyclotron resonance is formed to generate a high density plasma, and a grounded sample table is arranged in a position downstream of the plasma generating portion.
In the plasma generating methods described in Japanese Patent Application Laid-Open No. 7-288195 or Japanese Patent Application Laid-Open No. 7-297175 among the above-mentioned conventional technologies, a plasma is generated by a high frequency source of 13.56 MHz or several tens MHz. It is possible to generate a plasma appropriate for etching an oxide film under a gas pressure of several tens Pa to 5 Pa (Pascal). However, as a pattern dimension becomes as small as nearly 0.2 .mu.m or smaller, verticality in a processed shape is strongly required and consequently it is inevitable that the gas pressure decreases.
However, in the two-frequency exciting method or the M-RIE method described above, it is difficult to stably produce a plasma having a desired density higher than nearly 5.times.10.sup.10 cm.sup.-3 under a pressure condition lower than 4 Pa (0.4 to 4 Pa). For example, in the two-frequency exciting method described above, even if the plasma exciting frequency is increased up to a frequency around 50 MHz, the plasma density cannot be increased but, on the contrary, it decreases. Therefore, it is difficult to produce a plasma having a desired density higher than nearly 5.times.10.sup.-10 cm.sup.-3 under a pressure condition of 0.4 to 4 Pa.
Further, in the M-RIE method, the density distribution of a plasma generated by an action of confining electrons by Lorentz force of electrons produced on a surface of a sample must be uniform all over the surface of the sample. However, there is a disadvantage in that an inclination of the plasma density generally occurs over the surface of the sample due to drift of E.times.B. The inclination of the plasma density formed by the action of confining electrons cannot be corrected by any method such as diffusion or the like since the inclination occurs near the sheath in the vicinity of the sample where intensity of the magnetic field is strong.
Japanese Patent Application Laid-Open No. 7-288195 discloses a method of solving this problem in which it is possible to obtain a uniform plasma without inclination by arranging magnets so that the magnetic field intensity is weakened in a direction of electron drift due to the drift of E.times.B. even when a magnetic field with a maximum value as high as 200 gauss is applied in parallel to a sample. However, there is a disadvantage with this method in that it is difficult to follow a change in a processing condition since a condition for maintaining the plasma uniform is limited to a specified narrow range once the distribution of magnetic field intensity is fixed. In particular, in a case of a large sized sample having a diameter larger than 300 mm, when a distance between the electrodes is as narrow as 20 mm or less, pressure above the central portion of the sample becomes 10% or more greater than pressure above the peripheral portion of the sample. In order to avoid this pressure difference, the gap between the sample table and the opposite electrode must be set to 30 mm or more since, otherwise, the difficulty is likely to be increased.
As described above, in the two-frequency exciting method and the M-RIE method, it is difficult to obtain a uniform plasma density of 5.times.10.sup.10 cm.sup.-3 over the surface of a sample having a diameter of 300 mm or more under a pressure condition as low as 0.4 to 4 Pa. Therefore, in the two-frequency exciting method and the M-RIE method, it is difficult to manufacture the fine pattern of 0.2 .mu.m or smaller on a wafer having a diameter larger than 300 mm uniformly and quickly with a high selectivity of the etching material to the base material.
On the other hand, a method for substantially increasing a plasma density under a low pressure condition is disclosed in Japanese Patent Application Laid-Open No. 56-13480 among the prior art described above. However, this method has a disadvantage in that in a case where a silicon oxide film or a silicon nitride film is etched using a gas containing fluorine and carbon, it is difficult to attain a desired selectivity to the base material such as Si or the like since dissociation of the gas progresses excessively and a large amount of fluorine atoms and/or molecules and/or fluorine ions are generated. The ICP method using an electromagnetic field induced by an RF power source also has a disadvantage in that dissociation of the gas progresses excessively, the same as in the ECR method described above.
Further, the plasma processing apparatus is generally constructed in such a manner that the processing gas is exhausted from the peripheral portion of a sample. In such a case, there is a disadvantage in that the plasma density is higher in the central portion of the sample and lower in the peripheral portion of the sample, and accordingly uniformity in the processing all over the surface of the sample is degraded. In order to eliminate this disadvantage, a ring-shaped bank, that is, a focus ring is provided near the periphery of the sample to stagnate gas flow. However, there is another disadvantage in that reaction products attach onto the bank which becomes a particle producing source to decrease the product yield.
On the other hand, in order to control energy of ions incident to the sample, an RF bias with a sinusoidal waveform is applied to an electrode mounting the sample. The frequency of the RF bias used is several hundreds kHz to 13.56 MHz. However, the energy distribution of incident ions becomes of a double peak type. One of the two peaks is in a lower energy region and the other is in a higher energy region because the ions follow to change in electric field inside a sheath when the RF bias has a frequency within this frequency band. The ions in the higher energy range can process at high speed but damage the sample, and the ions in the lower energy range can process without damage but at low speed. That is, there is a disadvantage in that the processing speed is decreased when one tries to prevent damage of the sample, and the problem of damage arises when one tries to increase the processing speed.
On the other hand, when the frequency of the RF bias is set to a value higher than, for example, 50 MHz, the distribution of incident energy becomes of a single peak type. However, most of the energy is used in plasma generation and consequently the voltage applied to the sheath is substantially decreased. Therefore, there is a disadvantage in that it is difficult to control the energy of the incident ions independently to the plasma density.
Further, in the pulse bias power source method described in Japanese Patent Application Laid-Open No. 62-280378 or Japanese Patent Application Laid-Open No. 6-61182, there is no discussion of a case where a dielectric layer for electrostatic attraction is used between a sample table electrode and a sample while a pulse bias is applied to the sample. When the pulse bias method is directly applied to the electrostatic attracting method, an ion acceleration voltage applied between a plasma and the surface of the sample is decreased by the increase of the voltage generated between both ends of the electrostatic attracting film as ion current flows within one cycle of the RF bias, and consequently the distribution of ion energy is broadened. Therefore, the pulse bias power source method has a disadvantage in that it cannot cope with a required fine pattern processing while temperature of the sample is properly being controlled.
Further, in the conventional sinusoidal wave output bias power source method disclosed in the specification of U.S. Pat. No. 5,320,982, there is a disadvantage in that an impedance of the sheath portion approaches an impedance of the plasma itself or lower when the frequency becomes high. If this occurs, an unnecessary plasma is generated near the sheath in the vicinity of the sample by the bias power source, and accordingly the ions are not effectively accelerated and the distribution of the plasma is also degraded to lose controllability of ion energy by the bias power source.
Furthermore, in plasma processing, in order to improve the performance, it is important to properly control the amount of ions, the amount of radicals and the kinds of radicals. In the past, a gas to be formed into ions and radicals is introduced into a process chamber and the ions and the radicals are produced at the same time by generating a plasma in the process chamber. Therefore, as the processing of the sample becomes very small, it becomes clear that there is a limit in the control of the amount of ions, the amount of radicals and the kinds of radicals.
Further, in regard to an. example of utilizing cyclotron resonance of the VHF band, installation of a bias electric power source for applying a voltage to a sample table and a means for uniformly applying a voltage all over a sample surface are described in Journal of Applied Physics, Japan, Vol.28, No. 10. Further, a processing chamber has a height higher-than 200 mm. Therefore, the construction cannot use reaction on the surfaces of opposite electrodes effectively, and consequently it is difficult to obtain a high selectivity in this construction.